This bra size calculator finds your bra size from two measurements, in inches or centimetres. It shows your US, UK, EU and Australian sizes and your sister sizes, which have the same cup volume on a different band.
How to measure
- Underbust: wrap a soft tape snugly around your ribcage just below the bust, keeping it level.
- Bust: measure loosely around the fullest part of the bust, wearing a non-padded bra, with the tape level across your back.
How the size is worked out
- Band: the underbust in inches, rounded to the nearest whole number and up to an even number.
- Cup: each inch of difference between bust and band is one cup size: 1 = A, 2 = B, 3 = C, 4 = D and so on.
- US and UK sizes match up to D, then differ: UK uses DD, E, F, FF, G; US brands often use DD, DDD, G.
Cup size chart (difference in inches)
| Difference | US | UK |
|---|---|---|
| 1 | A | A |
| 2 | B | B |
| 3 | C | C |
| 4 | D | D |
| 5 | DD (E) | DD |
| 6 | DDD (F) | E |
| 7 | G | F |
| 8 | H | FF |
Sister sizes
Going down a band and up a cup keeps the same cup volume: 34C, 32D and 36B are sister sizes. Try them if the band feels too tight or too loose.
Frequently asked questions
How should a bra fit?
The band should be level and snug on the loosest hook, the centre should sit flat against your chest, and the cups should hold you without gaps or spilling.
Why is my size different between brands?
Brands use different patterns and stretch. Use the calculator as a starting point and check each brand’s size guide.
How often should I measure?
Every 6 to 12 months, or after weight change, pregnancy or breastfeeding.
What is the Australian size?
Australian band sizes are the UK band minus 22, so a UK 34 is an Australian 12. Cups follow UK letters.
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Last reviewed: October 2026. For information only; results are estimates; check important figures with a professional.